Indicator lamp control circuit employing charge-controlled timing capacitor coupling cascaded transistors



F. J. THOMAS ET AL 3,293,454 INDICATOR LAMP CONTROL CIRCUIT EMPLOYING CHARGE-CONTROLLED TIMING CAPACITOR Dec. 20, 1966 v COUPLING OASCADED TRANSISTORS 2 Sheets-Sheet 1 Filed March 16, 1964 INVENTOR,

FRANK J THOMAS THOMAS J. LA V/N HITORA/EY Dec. 20, 1966 THOMAS ET AL 3,293,454

INDICATOR LAMP CONTROL CIRCUIT EMPLOYING CHARGE-CONTROLLED TIMING CAPACITOR COUPLING CASCADED TRANSISTORS I 2 Sheets-Sheet 2 Filed March 16, 1964' 45 4- IZv 0V INPUT I |-MILLISECOND PERIOD OF INITIATING PULSE +25, CLAMPING VOLTAGE 77 COLLECTOR 67 11 4 v VOLTAGE AT BASE 9| DUE TO CHARGING OF CAPACITOR a7 +o.ev 111 fXSE 9| VOLTAGE AT BASE 9i DUE TO DISCHARGE OF CAPACITOR a7 COLLECTOR 95 E OF CQNDUCTION OF PERIOD TRANSISTOR 93 I v PERIOD OF ILLUMINATION -I INVEN'IOR.

United States Patent 3,293,454 INDICATOR LAMP CONTROL CIRCUIT EMPLOY- ING CHARGE-CONTROLLED TIMING CAPACI- TOR COUPLING CASCADED TRANSISTORS Frank J. Thomas, West Paterson, and Thomas J. Lavin,

Midland Park, N.J., assignors to The Bendix Corporation, Teterboro, N..I., a corporation of Delaware Filed Mar. 16, 1964, Ser. No. 351,920 13 Claims. (Cl. 30788.5)

This invention relates to improvements in an indicator lamp control circuit and more particularly to an improved time delay and driver circuit for controlling the illumination of a so-called Nixie bulb or neon lamp for use in indicating, for example, the operation of an engine of an aircraft under malfunction conditions, including novel means effective in response to a control electrical pulse of short duration for effecting illumination of the bulb for a much longer period.

Heretofore, it has been customary to use a so-called Nixie bulb or neon gas bulb bearing a cathode shaped arabic numeral to glow on illumination of the bulb so as to indicate a malfunction condition of an aircraft engine associated therewith.

An object of the invention is to provide a novel circuit means, the operation of which may be initiated by an electrical signal pulse of very short duration of, for example, .625 millisecond such as may be generated by a computing mechanism and to receive this short duration signal pulse to effect illumination of a Nixie bulb for a much longer duration of time, of for example, 500 milliseconds corresponding to the longest solution rate of the computer, so that the bulb may indicate a malfunction Condition sensed by the computer.

Another object of the invention is to provide in such an indicator lamp control circuit a first current control device for alternately effecting charging and discharging circuits for a capacitor, together with a second current control device for controlling a driver circuit for illuminating an indicator lamp, the first control device being rendered operative to effect the discharging circuit upon an electrical signal pulse being momentarily applied to a control element of the first control device and thereafter the first control device is rendered operative to effect the charging circuit for the capacitor upon cessation of th electrical signal pulse of short duration, and the second control device being rendered operative to complete the driver circuit for illuminating the indicator lamp for a much longer period of time dependent upon the duration of the flow of current in the charging circuit for the capacitor.

Another object of the invention is to provide a novel driver circuit for controlling the illumination of an indicator bulb of simplicity of construction and operative for eil'ecting a relatively long duration of illumination of the bulb in response to a control pulse of much shorter duration.

These and other objects 'and features of the invention are pointed out in the following description in terms of the embodiment thereof which is shown in the accompanying drawings. It is to be understood, however, that the drawings are for the purpose of illustration only and are not a definition of the limits of the invention. Reference is to be had to the appended claims for this purpose.

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In the drawings:

FIGURE 1 is a wiring diagram of an illuminating bulb driver circuit embodying the present invention.

FIGURE 2 is a graphical illustration showing the relative duration of the initiating pulse for efiecting operation of the lamp illuminating circuit, indicated graphically by the bar A, and the greater period of illumination of the lamp in response to the initiating pulse, indicated graphically by the bar B as well as the relation of the several voltage wave forms, i.e. at I the voltage wave form of a typical input voltage; at H the resulting voltage wave form at collector 67; at III the resulting voltage wave form at base 91; at IV the resulting voltage wave form at collector 95 together with the period of conduction of the transistor 93; and at V the period of illumination of the Nixie bulb 122.

Referring to the drawing of FIGURE 1, there is indicated diagrammatically by the numeral 16 a suitable mechanism such as a computer adapted to control a switch 12 arranged to close a grounded contact 14 under normal operating conditions of an engine or other device sensed by the mechanism 1% However, upon a sensed malfunction occurring in the engine, the computer or device 10 is effective to momentarily actuate the switch 12 out of closing relation with the grounded contact 14 to effect the control function, as hereinafter described.

The switch 12, shown diagrammatically herein, may be an electronic valve, transistor, logic circuitry, or a suitable current control device of a type well known in the art to eifect the desired control operation. The switch 12 is connected through a conductor 38 and resistor element 40 to the positive terminal of a battery having its negative terminal grounded at 47 to complete a first circuit for the battery 45 effective under normal operating conditions.

Further, connected to the conductor 38, there is provided a diode 49 arranged to permit the passage of current from the positive terminal of the battery 45 to a conductor 51 upon the switch 12 opening the grounded contact 14, as upon a malfunction condition being sensed by the computer or device 10.

However, under normal operating conditions, the flow of current from the positive terminal of the battery 45 is through the switch 12 to the grounded contact 14 and is thereby diverted from the diode 49 and conductor 51.

The conductor 51 is connected thnough a resistor 53 to a negative terminal of a battery 55 having its positive terminal grounded at 57 so as to complete a second circuit for batteries 4-555 effective upon the switch 12 opening the grounded contact 14. A point 59 on conductor 51 between the resistor 53 and diode 49, is connected by a conductor 61 to a control or base element 63 of a current control device or transistor 65 having a collector element 67 and an emitter element 69.

The emitter element 69 is connected to ground by a conductor 71 while the collector element 67 is connected by a conductor 72 to a clamping diode 73 which is in turn connected through a conductor 75 to the positive terminal of a battery 77 having its negative terminal grounded at '79. The clamping diode 73 is arranged to block the passage of current from the positive terminal of the battery 77 to the conductor 72.

Further, a conductor leads from the collector 67 to a plate of a capacitor 87 while an opposite plate of the capacitor 87 is connected by a conductor 89 to a control or base element 91 of a second current control device or transistor 93 having a collector element 95 and an emitter element 97. The emitter element 97 is connected through a conductor 99 'to ground while the collector element 95 is connected through a conductor 101 to a conductor 103, resistor element 105 and conductor 107 to the conductor 72.

Also, leading from the conductor 101 is a conductor 110, resistor 112, and conductor 114 connected to a positive terminal of a battery 116 having a negative terminal thereof connected by a conductor 118 to ground.

Also, leading from the positive terminal of the battery 116 is a conductor 121 connected to one electrode of a so-called Nixie" bulb or neon gas indicator lamp 122 which may be of a conventional type having an opposite electrode connected through a resistor 124 to the conductor 191.

Further leading from the conductor 89 is a resistor element 130 which has its opposite terminal connected to ground through a conductor 132 so as to serve with the base and emitter elements of the transistor 93 to complete a charging circuit for the capacitor 87 from the battery 116, as hereinafter explained.

There is also connected to the conductor 89 one terminal of a recovery diode 134 having its opposite terminal connected to ground through a conductor 136. The recovery diode 134 is arranged to block the passage of positive current from the conductor 89 to the grounded conductor 136 while permit-ting the passage of positive pulses of current from the ground conductor 136 to the conductor 89 so as to complete a discharging circuit for the capacitor 87 through the transistor 65 opon the transistor 65 being in an on condition.

In the operation of the circuit of FIGURE 1, a malfunction condition sensed by the computer or control mechanism causes the switch 12 to be opened or lifted momentarily from the grounded contact 14 for a period of an extremely short duration such as, for example, .625 millisecond causing current to flow from the positive terminal of the battery 45 through the resistor 40, conductor 38, diode 49, conductor 51, and resistor 53 to the negative terminal of the battery 55 and thereby through the grounded conductors 57 and 47 to the battery 45.

A voltage drop across resistor 53 is thereupon effective at the point 59 to cause a positive pulse to be applied through the conductor 61 to the base element 63 causing the transistor 65 to saturate and the capacitor 87 to discharge positive going current from the positively charged plate thereof through the conductor 85, collector element 67, the transistor 65, emitter 69, and grounded conductor 71 returning through grounded conductor 136, recovery diode 134, and conductor 89 to the opposite negatively charged plate of the capacitor 87. Upon completing the duration of the extremely short malfunction signal of, for example, .625 millisecond, the switch 12 is returned to the condition closing the grounded contact 14 so that the current from the battery 45 is diverted from the diode 49 through the grounded connection of the contact 14 whereupon the positive pulse applied by the battery 45 at the point 59 and through conductor 61 to the base element 63 is removed and replaced by a negative charge applied through the resistor 53 by the battery 55. The transistor 65 thereupon returns to an off condition and the capacitor 87 starts to be positively charged at the plate connected to conductor 85 through resistors 105 and 112 from the positive terminal of battery 116 while the opposite plate of the capacitor 87 connected to conductor 89 is in turn connected through the base and emitter elements of the transistor 93 as well as resistor 130 and grounded conductors 99, 132 and 118 to the negative terminal of battery 116.

applied through the conductors 72 and 85 to a predetermined value set by the clamping diode 73.

The charging action of the battery 116 on the capacitor 87 serves to positively charge the capacitor plate connected to conductor 85 and in so charging the capacitor 85 causes current to flow from the opposite plate of the capacitor 87 through conductor 89. The current from conductor 89 flows into the base element 91 of the transistor 93 and from the emitter element 97 to the grounded conductor 99 as Well as through resistor 130 and grounded conductors 132 and 118 to the negative terminal of the battery 116 whereupon the voltage drop across resistor 130 1 causes a positive going pulse to be applied at the base element 91 of the second transistor 93 turning the transis-tor 93 on for the duration of the charging period of the capacitor 87.

The base and emitter elements of the transistor 93 serve to etfectively clamp the voltage across resistor 130 acting The potential of the battery 116 may be, for example,

in a positive voltage sense in the flow of current from the conductor 89, while the diode 134 serves to effectively clamp the voltage across resistor 130 acting in an opposite or negative voltage sense in the flow of current toward the conductor 89 from the grounded conductor.

The turning on of the transistor 93 causes current to flow from the positive terminal of the battery 116 through conductor 121 to the so-called Nixie bulb or lamp 122, resistor 124 and through transistor 93 to the grounded connection 99 leading to the grounded conductor 118 and thereby returning to the negative terminal of the battery 116 so as to cause the Nixie bulb to be illuminated.

The voltage diiferentials across the electrodes of the lamp 122 under the control of the transistor 93 are thus increased to a magnitude to render the lamp 122 conductive and effect the illumination thereof.

The charging current to capacitor 87 continues to flow and the positive charge resulting from the voltage drop across resistor 130 continues to be applied to the base element 91 of the transistor 93 for a period dependent upon the effective time constant of the circuit including the components indicated by numerals 114, 112, 110, 103, 105, 107, 72, 85, 87, 89, 130, 132 and 118.

The effective time constant is considerably increased as a result of a positive current feedback provided through resistor 105 in response to the transistor 93 being first made conductive and then progressively made less conductive to effect, respectively, first a maximum low level voltage at the collector which is thereafter progressively raised to an increasing voltage level at collector 95. The rate of decay of the base current into transistor 93 is reduced by the action of the positive current feedback resulting from the progressive increase in the voltage level at the collector 95 which acts in a sense tending to maintain a constant flow of charging current to the capacitor 87.

Thus the initial opening of the first transistor 65 renders effective a series charging circuit including resistors 112, and 130, capacitor 87 and the base and emitter elements of the transistor 93 so as to provide an initial current level sufiicient to render the transistor 93 conductive between the collector 95 and the emitter 97 to elfect energization of the Nixie bulb 122.

This initial conductive action of the transistor 93 further eifects a sharp drop in the potential at the collector element 95 of the transistor 93 to in turn sharply decrease the flow of current applied through the resistor 105 in the charging circuit. After which the flow of current to the base 91 of the transistor 93 in the charging circuit is thereby decreased which in turn causes the transistor 93 to become less conductive between the collector and the emitter to effect an increase in the potential at the collector 95. This in turn will effect a flow of current in the charging circuit of the capacitor 87. The flow of current from the capacitor 87 to the base 91 being decreased as the charge applied to the capacitor 87 approaches the potential at the collector 95 to in turn progressively decrease the conduction of the transistor 93 to raise the potential at the collector 95 until the capacitor 87 is fully charged whereupon the transistor 93 is then rendered non-conductive. The Nixie bulb- 122 is deenergized at some portion of the output voltage ramp effected at collector 95 dependent upon the operating characteristics of the particular Nixie bulb 122 that may be used, as shown graphically at IV and V of FIG- URE 2. Thus, during the charging of the capacitor 87 there is effected through the operation of the transistor 93 in the charging circuit, a positive current feedback through the resistor 105 (negative rate voltage feedback) so that the time constant of the charging circuit for the capacitor 87 is effectively increased.

Through a proper selection of the charging circuit parameters, the Nixie bulb 122 may be illuminated for a much longer period, indicated graphically in FIG- URE 2 by the bar B, of for example 500 milliseconds, than that of the initiating momentary signal pulse, indicated graphically in FIGURE 2 by the bar A, of for example, .625 millisecond.

It will be seen then that upon the voltage applied at the collector 67 of the first transistor 65 and through the resistor 105 by the variable potential at the collector 95 of the second transistor 93 being raised to the voltage level effected through the clamping diode 73 by the battery 77, the flow of current ceases from the negatively charging plate of the capacitor 87 whereupon the positive bias applied to the base 91 of the transistor 93 due to the voltage drop across the resistor 130 ceases. The transistor 93 is thereupon turned off or rendered non-conductive, as indicated graphically by FIGURE 2.

The illumination of the Nixie bulb 122 is effected upon the voltage level at the collector 95 being decreased to below a predetermined value determined by the operating characteristics of the Nixie bulb 122 and thereafter the bulb 122 is deenergized upon the voltage level at the collector 95 being increased to the predetermined value at which value the voltage drop across the bulb 122 is insufficient to cause illumination, as heretofore explained with reference to the graphical illustration of FIGURE 2.

Through the novel arrangement of the circuit herein described, there may be effected, as shown graphically in FIGURE 2, an initiating signal pulse of very short duration, for example, .625 millisecond and which short duration effective signal pulse may cause through the circuit an illumination of the Nixie bulb 122 or neon gas bulb for a much longer duration of time of, for example, 500 milliseconds. The circuit for effecting this operation is of simple construction and may be readily produced at low cost for use where minimized circuitry is of prime importance.

Although only one embodiment of the invention has been illustrated and described, various changes in the form and relative arrangement of the parts, which will now appear to those skilled in the art may be made without departing from the scope of the invention. Reference is, therefore, to be had to the appended claims for a definition of the limits of the invention.

What is claimed is:

1. The combination comprising a first source of electrical energy, a first circuit energized from said first source including a first resistor and a control means for opening and closing the first circuit, a second circuit for the first source of electrical energy including a diode and a second resistor, the second circuit being rendered ineffective upon the closing of the first circuit by the control means and effective upon the opening of the first circuit by the control means, a first current control device including means responsive to a voltage drop across the second resistor to render the first current control device effective during a flow of current through the second circuit, a timing capacitor, a discharging circuit for the timing capacitor including the first current control device, a charging circuit for the timing capacitor operative upon the first current control device being rendered ineffective, the charging circuit for the timing capacitor including a third resistor, -a second current control device including means responsive to a voltage drop across said third resistor to render the second control device operative for effecting a control function during a flow of current in said charging circuit.

2 The combination defined by claim 1 including an indicator lamp, a control circuit for the indicator lamp including said second current control device operably connected in said control circuit, said second current control device being operable in one sense to render said control circuit effective to illuminate said indicator lamp in response to the voltage drop across said third resistor, and said second current control device being thereafter operable in an opposite sense in response to a decay in the voltage drop across said third resistor to effect a current feedback in the charging circuit for the capacitor to decrease the rate of the decay in the voltage drop across said third resistor.

3. In a time delay circuit controlling apparatus com- I prising a first transistor having a collector, an emitter, and a base, a capacitor, a charging circuit for the capacitor operatively connected to the collector of the first transistor, a discharging circuit for the capacitor including the collector and the emitter of the first transistor, said discharging circuit including a recovery diode effectively connecting the emitter of the first transistor to one plate of the capacitor for permitting discharging current to flow from the opposite plate of the capacitor to said one plate of the capacitor while blocking flow of charging current therethrough, the charging circuit for the capacitor being rendered operative upon the first transistor being ineffective to permit a flow of current therethrough from the collector to the emitter, said charging circuit including a source of electrical energy and a resistor, a second transistor having a collector, an emitter, and a base, the collector and the emitter of the second transistor being connected in the time delay circuit, the 'base and the emitter of the second transistor being connected across the resistor of the charging circuit to render the second transistor effective to permit a flow of current through the time delay circuit in response to a voltage drop across said resistor of the charging circuit during a flow of current in said charging circuit, means for momentarily applying a signal voltage of short duration to the base of the first transistor to render the same effective to permit the flow of current therethrough from the collector to the emitter of the first transistor to discharge the capacitor through the discharging circuit, and upon cessation of said momentary signal voltage the first transistor renders the discharging circuit ineffective and the charging circuit for the timing capacitor effective, whereupon the second transistor is effective to permit a flow of current through the time delay circuit for a period of relatively long duration dependent upon flow of current in the charging circuit.

4. In a time delay circuit, a control apparatus comprising a first current control device, a capacitor, a charging circuit for the capacitor, a discharging circuit for the capacitor, said first current control device including condition responsive means operative in one sense to cause said first control device to render the discharging circuit effective to discharge the capacitor, said condition responsive means operative in another sense to cause said first control device to render said discharging circuit ineffective and said charging circuit effective to charge the capacitor, a second current control device, said second control device including means responsive to floW of current in said charging circuit to operate the second control device, the time delay circuit including said second concontrol device, said time delay circuit being rendered effective upon the operation of the second current control device in one sense, and means electrically connecting the second control device to the charging circuit to effect a feedback current flow in the charging circuit to the capacitor in response to operation of the second control device in another sense immediately following operation of the second control device in said one sense.

5. The combination defined by claim 4 in which the charging circuit includes a resistor, and the means for rendering the second current control device operative in said one sense includes'means responsive to a voltage drop across said resistor during flow of current in said charging circuit.

6, The combination defined by claim 4 in which the discharging circuit includes a recovery diode for permitting discharging current from the capacitor to flow from one plate of the capacitor to an opposite plate thereof while said diode acts to block flow of charging current therethrough.

7. The combination defined by claim 6 in which the charging circuit includes a resistor, and the means for rendering the second current control device operative in said one sense and in said other sense includes means responsive to a voltage drop across said resistor during flow of current in said charging circuit.

8. In combination, a first current control device having an anode, a cathode, and a control electrode; means including a capacitor cooperative with said control electrode and including a timing network which maintains said first device in a conductive condition between said anode and cathode during an interval in which the capacitor charges and which timing network after a predetermined time interval renders said first device nonconductive between said anode and cathode upon completion of the charging of said capacitor, a second current control device having an anode, a cathode, and a control electrode, a discharging circuit for the capacitor including the anode and cathode of said second control device, a third current control device cooperative with the control electrode of said second device, said third device including means operative in a first sense to effect the control electrode of said second device in a sense to normally maintain said second device nonconductive between the anode and cathode thereof, said last-mentioned means of said third device being momentarily operative in a second sense to change the effect of the control electrode of the second device in a sense to render said second device conductive between the anode and cathode thereof to complete the discharge circuit for the capacitor, and said last-mentioned means of said third device being thereafter operative in said first sense to render said second device nonconductive between the anode and cathode thereof to initiate the charging of the capacitor and the maintenance of said first device in the conductive condition between said anode and cathode during the interval in which the capacitor charges.

9. The combination defined by claim 8 in which said timing network includes a variable current feedback means for charging said capacitor, means operatively connecting the anode of said first current control device to said feedback means to vary the feedback current to said capacitor in inverse relation to the conductive condition between said anode and cathode of the first current control device during the charging interval of the capacitor.

10. A time delay circuit controlling apparatus comprising a first current control device having an anode and a cathode connected in said time delay circuit, and a control electrode, a second current control device having an anode, a cathode and a control electrode, a capacitor, charging and discharging circuits for said capacitor, said capacitor being connected in said charging circuit and between the anode of the second device and the control electrode of the first device to render the first device conductive between the anode and cathode thereof during the charging of the capacitor, a source of electrical energy for said charging and time delay circuits, said charging circuit including a first resistor connected between the anode of said first device and the anode of said second device, the source of electrical energy having a positive terminal connected to the anode of the first device and a negative terminal connected to the cathode of the first device, a second resistor connected in said charging circuit between the negative terminal of the source of electrical energy and the control electrode of the first device, means connecting the anode and the cathode of said second control device in the discharging circuit of the capacitor, control means for the control electrode of said second control device, said control means being momentarily operable in one sense to render the second control device conductive between the anode and cathode thereof so as to initiate operation of the discharging circuit, and said control means being thereafter operable in another sense to render the second control device nonconductive between the anode and cathode thereof to terminate operation of the discharging circuit and initiate operation of the charging circuit to render the time delay circuit effective for a predetermined time interval.

11. A time delay circuit controlling apparatus comprising a first current control device having an anode and a cathode connected in said time delay circuit, and a control electrode, a second current control device having an anode, a cathode and a control electrode, a capacitor, charging and discharging circuits for said capacitor, said capacitor being connected in said charging circuit and between the anode of the second device and the control electrode of the first device to render the first device conductive between the anode and cathode thereof during the charging of the capacitor, said second device having the anode and cathode thereof connected in said discharging circuit, and means for selec tively applying a signal voltage to the control electrode of the second device to render the second device conductive between the anode and cathode thereof to initiate operation of the discharging circuit and thereafter nonconductive to initiate operation of the charging circuit for the capacitor to render the time delay circuit eiTective for a predetermined time interval.

12. In a time delay circuit controlling apparatus of a type including a current control device having an anode and a cathode connected in said time delay circuit and a control electrode, a capacitor, charging and discharging circuits for said capacitor, and control means for selectively rendering the discharging and charging circuits effective and ineffective; the improvement comprising means operatively connecting the control electrode in the charging circuit so as to render the current control device conductive between the anode and cathode in response to current flow in the charging of the capacitor, the charging circuit including a voltage feedback means operable by the current control device, the conductivity of the current control device between the anode and cathode during the charging of the capacitor varying the voltage feedback in an inverse relation to the conductivity of the control device for rendering the time delay circuiteffective for a predetermined time interval after the rendering of the discharging circuit ineffective, and the control means being selectively operable in one sense to render the discharging circuit momentarily efiective to discharge the capacitor and thereafter operable in another sense to initiate operation of the charging circuit.

13. In a time delay circuit controlling apparatus of a type including a current control device having an anode and a cathode connected in said time delay circuit and a control electrode, a capacitor, and charging and discharging circuits for said capacitor; the improvement comprising means operatively connecting the control electrode in the charging circuit so as to render the current control device conductive between the anode and cathode in response to current flow in the charging of the capacitor, the conductivity of the current control device between the anode and cathode during the charging of the capacitor rendering the time delay circuit effective for a predetermined time interval, control means selectively operable in one sense to render the discharging circuit momentarily efiective to discharge the capacitor and thereafter operable in another sense to initiate operation of the charging circuit, said charging circuit including a variable current positive feedback means for charging said capacitor, means operatively connecting said current control device to said positive feedback means to vary the positive feedback current to said capacitor in inverse relation to the conductive condition between said anode and cathode of the current control device during the charging of the capacitor.

References Cited by the Examiner UNITED STATES PATENTS 10 ARTHUR GAUSS, Primary Examiner.

J. S. HEYMAN, Assistant Examiner. 

1. THE COMBINATION COMPRISING A FIRST SOURCE OF ELECTRICAL ENERGY, A FIRST CIRCUIT ENERGIZED FROM SAID FIRST SOURCE INCLUDING A FIRST RESISTOR AND A CONTROL MEANS FOR OPENING AND CLOSING THE FIRST CIRCUIT, A SECOND CIRCUIT FOR THE FIRST SOURCE OF ELECTRICAL ENERGLY INCLUDING A DIODE AND A SECOND RESISTOR, THE SECOND CIRCUIT BEING RENDERED INEFFECTIVE UPON THE CLOSING OF THE FIRST CIRCUIT BY THE CONTROL MEANS AND EFFECTIVE UPON THE OPENING OF THE FIRST CIRCUIT BY THE CONTROL MEANS, A FIRST CURRENT CONTROL DEVICE INCLUDING MEANS RESPONSIVE TO A VOLTAGE DROP ACROSS THE SECOND RESISTOR TO RENDER THE FIRST CURRENT CONTROL DEVICE EFFECTIVE DURING A FLOW OF CURRENT THROUGH THE SECOND CIRCUIT, A TIMING CAPACITOR, A DISCHARGING CIRCUIT FOR THE TIMING CAPACITOR INLCUDING THE FIRST CURRENT CONTROL DEVICE, A CHARGING CIRCIRCUIT FOR THE TIMING CAPACITOR OPERATIVE UPON THE FIRST CURRENT CONTORL DEVICE BEING RENDERED INEFFECTIVE, THE CHARGING CIRUCIT FOR THE TIMING CAPACITOR INCLUDING A THIRD RESISTOR, A SECOND CURRENT CONTROL DEVICE INCLUDING MEANS RESPONSIVE TO A VOLTAGE DROP ACROSS SAID THIRD RESISTOR TO RENDER THE SECOND CONTOL DEVICE OPERATIVE FOR EFFECTING A CONTOL FUNCTION DURING A FLOW OF CURRENT IN SAID CHARGING CIRUIT. 